The research described in this proposal will focus on a quantitative analysis of the processing of the tastes of salts and acids, employing the electrophysiological characterization of the responses of single cells in the nucleus of the solitary tract (NST) of the hamster, behavioral studies of taste similarity in the hamster, and human psychophysical experiments. Sensitivities arising from the anterior tongue produce classes of sucrose-, NaCl-, HC1-, and occasionally, QHC1-best neurons in the NST; multivariate analyses suggest that these classes comprise distinct sets of response profiles. These cells are not specific for their best stimuli and their breadth of tuning increases through the brainstem relays. New insights into the organization of gustatory sensitivities in the afferent pathway come from recent data showing that Na(+) sensitivity arising from amiloride-sensitive membrane channels on taste receptor cells is segregated into a single class of afferent neurons (NaCl-best) and that this segregation is maintained in 2nd-order cells of the NST. The proposed experiments will address the organization of taste sensitivities in NST cells by employing stimuli which should have relatively specific effects on proposed Na(+) and H(+) receptor mechanisms. An array of these stimuli plus some sweet- and bitter-tasting compounds, will be matched for their effectiveness on integrated multiunit NST activity and used at these single matched concentrations to study the responsiveness of NST cells. By using equally effective stimuli, rather than concentrations that are midrange for each stimulus, these studies will more rigorously test the hypothesis that the profiles of these cells are discretely organized. The responses of NST cells will be tested following amiloride treatment of the tongue or after adaptation to sodium salts or acids in an effort to parcel out the sensitivities represented in each neuron type. Behavioral experiments using the generalization of conditioned taste aversions in hamsters with IXth nerve transections will characterize the taste similarities and differences among these stimuli that are mediated via VIIth nerve taste fibers. A parallel series of human psychophysical studies will examine the effects of amiloride on magnitude estimates of the saltiness of sodium and non-sodium salts, including organic salts. Cross-adaptation experiments will address the specific effects of NaCl adaptation on the saltiness of the same stimuli employed in the electrophysiological experiments in an attempt to relate the specific effects of adaptation on human sensation to those seen in the electrophysiological studies and to those produced by amiloride. By examining the relationships among receptor mechanisms, sensitivity profiles of gustatory afferent neurons, hamster behavioral measures of taste similarity, and human judgements of taste quality, the proposed experiments will provide new insights into the neural coding of gustatory information.